MEMS are very small integrated systems that combine mechanical and electrical components, which traditionally range in size from the micrometer to the millimeter level.
The difficulty in controlling surface forces existing inside inertial MEMS devices is a critical obstacle to their fabrication and using. In particular, surface phenomena such as stiction of two micro-surfaces facing one another frequently restrict the operational environment and limit the lifetime of these devices.
By definition, stiction phenomenon occurs when surface adhesion forces (or stiction forces) are higher than mechanical restoring forces of the micro-surfaces.
In addition, with the decrease in the dimension of MEMS microstructures over recent years, this surface phenomenon has become more and more effective.
A well-known problem is in-use stiction that appears during operation and handling of inertial MEMS devices when micro-surfaces, such as conductive electrodes, come into contact and permanently adhere to each other, causing MEMS failure. In-use stiction may be caused by the following stiction forces: capillary, electrostatic (or Van Der Walls) and chemical bonding forces. Those stiction forces essentially depend upon nature of the used materials in the inertial MEMS device, surface topography and surface treatment method.
One particular structure of such an inertial MEMS device, as illustrated in FIG. 1, consists for example of a mobile mass 150 comprising one mobile electrode, suspended by springs means 115 to an armature (not shown), said mobile electrode comprising a plurality of conductive fingers 155. The inertial MEMS device further comprises two sets of two fixed electrodes 120a, 120b, rigidly attached to the armature, each fixed electrodes comprising two conductive fingers 125a, 125b. Each conductive finger 125a, 125b faces one of the mobile mass fingers 155 to form a pair of conductive fingers (with associated capacitance value) that is used to move down or up the mobile mass along a sensitive axis 130.
Here below, “spring means” (also called “flexible beam”) is understood to be every flexible connection means for elastically relying the mobile mass to the armature.
The technical problem of reducing probability of finger stiction in-use of an inertial MEMS device is a problem known by a person skilled in the art and several techniques have been provided to solve it.
A first known technique consists in rising restoring forces of springs 115 by rising spring stiffness and/or mobile mass excursion. But to obtain higher spring stiffness, inertial MEMS developers are forced to conceive more massive springs, which makes the inertial MEMS device less compact. Also, if spring stiffness increases, sensitivity of the inertial MEMS device and so signal-to-noise ratio (or SNR) are reduced.
Furthermore, even for an inertial MEMS device comprising such massive springs, in-use stiction phenomenon still persists.
A second known technique consists in reducing stiction forces by means of a suitable coating of surfaces of the conductive fingers which are susceptible to come in contact, such a coating—as known as “anti-stiction coating”—being made of a low-energy surface material and/or a high-roughness surface material.
However, low-energy surface coatings require a surface treatment process, which has the well-known drawbacks of complexity of implementation, manufacture and of cost.
In addition, even for conductive finger surfaces treated with an anti-stiction coating, in-use stiction phenomenon is still of concern.
Therefore, one common drawback of these two aforesaid prior-art techniques is that they are not sufficiently effective, since they do not ensure that the problem of MEMS in-use stiction is totally eliminated. It therefore becomes impossible to ensure to a user that his inertial MEMS device will not be in a non-functioning state.
To overcome this drawback, it may be a common practice to apply one or several mechanical shock(s) to the MEMS device according to its sensitive axis, for example by means of a vibration system, in order to add an additional force component to the restoring force of springs so as to generate a restoring force higher than the stiction forces. This practice can be complex to implement and cost a lot.
A third known technique, as illustrated in the patent document US 2007/075942, consists in applying a predefined voltage between the conductive electrodes of the MEMS device, so as to create an electrostatic force that generates a displacement of the mobile mass according to a direction opposite to the direction of the stiction force, thereby leading to a separation of the stuck conductive electrodes.